1. Field of the Invention
The invention relates to a controller for a driving force transmitting apparatus mounted in a four-wheel-drive vehicle.
2. Description of the Related Art
A four-wheel-drive vehicle includes a pair of right and left main drive wheels to which a driving force generated by a drive source is always transmitted while the four-wheel-drive vehicle is traveling and a pair of right and left sub-drive wheels to which a driving force generated by the drive source is transmitted so as to be adjustable based on a traveling state. Conventionally, such a four-wheel drive vehicle includes a driving force transmitting apparatus configured to transmit the driving force to the sub-drive wheels and a controller configured to control the driving force transmitting apparatus. Such a controller may have a function of determining whether a road surface is in a low-μ (low friction) condition or a high-μ (high friction) condition, and may be configured to control the driving force transmitting apparatus based on the determination result. See, for example, Japanese Unexamined Patent Application Publication No. 2004-338685 (JP 2004-338685 A).
A controller (driving force distribution controller) described in JP 2004-338685 A calculates an amount of low-μ information associated with a coefficient of road surface friction based on a slip ratio of a wheel and an amount of acceleration operation. When the amount of low-μ information is larger than an addition-determination threshold value, the controller adds the amount of low-μ information to a count value. On the other hand, when the amount of low-μ information is equal to or smaller than the addition-determination threshold value, the controller subtracts a constant from the count value. When the count value is larger than a low-μ road determination threshold value, the controller determines that the road surface is in the low-μ condition.
The controller described in JP 2004-338685 A calculates the amount of low-μ information associated with the coefficient of road surface friction based on the slip ratio of the wheel and the amount of acceleration operation. Thus, when the amount of acceleration operation is small, for example, when the vehicle is in a steady traveling state where the vehicle is travelling straight forward on a flat road surface at a constant speed, the controller may fail to accurately determine a road surface condition.